1. Field of the Invention
The present invention relates to a contact material of excellent current chopping characteristic and high voltage withstanding characteristic.
2. Description of the Related Art
For example vacuum interrupter contacts are constructed of various raw materials in order to maintain and improve the three basic requirements represented by anti-welding interrupting characteristic, voltage withstanding characteristic, and current interrupter characteristic, and, in addition, current chopping characteristic, erosion characteristic, contact resistance characteristic, and temperature rising characteristic etc.
However, since, for the characteristics demanded above, mutually contradictory material properties are typically required, these cannot be fully satisfied by a single element. Accordingly, the present situation is that contact materials adapted to specific applications such as large current interrupter characteristic applications, high voltage withstanding characteristic applications, or low current chopping characteristic applications are being developed by such measures as use of composite materials or cladding separate members together, and these have excellent characteristics in their own way.
As contact materials for large current interruption for satisfying the basic three requirements for ordinary vacuum circuit breakers, there are known for example Cu--Bi alloy and Cu--Te alloy containing up to 5 weight % of anti-welding constituents such as Bi or Te (Issued Japanese Patent No. Sho.41-12131 and Issued Japanese Patent No. Sho.44-23751). In the case of Cu--Bi alloy, the brittle Bi, Cu--Te alloy that is precipitated at the grain boundaries produces embrittlement of the grain boundaries and the brittle Cu--Te that is precipitated within the particles produces embrittlement of the alloy itself; as a result, a low weld pull-apart force is realized, enabling an excellent large current interrupter characteristic to be achieved. Of these alloys, contacts in which the Bi content is for example around 10 weight % have suitable vapor pressure characteristics, so they exhibit excellent current chopping characteristics (Issued Japanese Patent No. Sho.35-14974). Cu--Cr alloy is known as a contact material for contacts likewise satisfying the three basic requirements, having high voltage withstanding characteristics and a large current interrupter characteristic. This alloy has the advantage that it can be expected to show more uniform performance than the Cu--Bi alloy or Cu--Te alloy mentioned above since the vapor pressure difference between its constituents is small and, depending on the way in which it is used, shows excellent performance.
On the other hand, in recent years, it has become necessary to further improve the current chopping characteristic and voltage withstanding characteristic (restriking characteristic) of vacuum interrupters intended to be of high-reliability, miniaturized type.
Firstly, vacuum interrupter contacts, in which current chopping (or current switching) is performed under high vacuum by utilizing the dispersion of an arc in vacuum, are constructed of a pair of fixed and movable contacts arranged facing each other. If, when used in an inductive circuit such as an electric motor load, current is interrupted without sufficient care regarding the vacuum interrupter, an excessive abnormal surge voltage is generated, which may affect the insulating characteristics of the load equipment. The reasons for occurrence of this abnormal surge voltage include the phenomenon of chopping occurring on the low-current side (i.e. current interruption is performed forcibly without waiting for the natural zero point of the AC current waveform) when performing small-current interruption in vacuum, or the phenomenon of high frequency arc extinction. The value Vs of the abnormal surge voltage is proportional to the surge impedance Z.sub.0 of the circuit and the current chopping value Ic. As one means of suppressing the abnormal surge voltage value Vs to a low level, it is therefore necessary to make the current chopping value Ic low. In this connection, Ag--WC alloy can be utilized as one type of contact alloy that is advantageous in respect of this demand.
Ag--WC alloy (Ag 40%) is known as an example of such a low chopping characteristic contact material, and has an excellent low chopping characteristic on account of the synergetic action of the thermion discharge effect of WC and the appropriate vapor pressure of Ag (Japanese Patent Application No. Sho.42-68447). It is also suggested that benefits can be obtained in improving the current chopping characteristic by using contact material in which the particle size of the anti-arcing constituent material (for example the particle size of WC) is 0.2.about.1 .mu.m (Issued Japanese Patent No. H.5-61338). In addition, contact materials are also known (Early Japanese Patent Publication No. H.4-206121) in which a large current interrupter characteristic is improved by obtaining good mobility of the arc cathode point by the use of a contact material in which the inter-particle separation of WC--Co particles is chosen to be 0.3.about.3 .mu.m.
Secondly, the phenomenon of the occurrence of a conductive condition (subsequent discharge does not continue) between the electrodes may be produced in a vacuum interrupter by occurrence of flashover within the vacuum interrupter after current interruption. This phenomenon is called restriking. Although the mechanism of its occurrence is not understood, it can easily generate abnormal overvoltage, since there is an abrupt change to a conductive condition after the electrical circuit has first been put into a current interrupting condition. Even in the case of an interrupter using Ag--WC alloy, whose current interrupting characteristics are excellent, according to tests in which restriking was produced by interrupting a capacitor bank, occurrence of very large overvoltage and/or occurrence of excessive high frequency currents were observed. Development of a technique for suppressing occurrence of restriking for contacts using Ag--WC alloy is therefore sought. The mechanism of occurrence of the restriking phenomenon of Ag--WC alloy is as yet unknown but according to the experimental observations of the present inventors restriking occurs with fairly high frequency between one contact and another contact in vacuum interrupters and between the contacts and the arc shield. The present inventors were therefore able to achieve a contribution to suppression of occurrence of restriking by showing that techniques for suppression of abrupt gas that is released for example when the contacts are subjected to arcing and techniques for optimization of the form of the contact surface are very effective in suppressing occurrence of restriking. Specifically, by noting the total amount of gas released in the process of heating the Ag--WC alloy, the type of gas and its mode of ignition, and observing the inter-relationship with occurrence of restriking, they discovered that contacts in which a large amount of gas is abruptly released in the form of a pulse, even though only for a very short time, in the vicinity of the melting point, show a high rate of occurrence of restriking. Accordingly, occurrence of restriking was reduced by removing beforehand one of the causes of release of abrupt gas in the Ag--WC alloy by for example heating to above the melting point of Ag, or by improving sintering technology such as to suppress pores or structural segregation in the Ag--WC alloy. However, the need for further improvement is recognized and it has become important to develop other measures to meet recent demands for further suppression of restriking. In recent years, the diversity of loads has increased and conditions of use demanded by users have shown a marked tendency to become more severe, including for example application to reactor circuits and capacitor circuits and demands have increased for a low current chopping Ag--WC alloy with even lower chopping characteristics and even lower restriking characteristics, and the development and improvement of contact materials in this respect has become an urgent task. In capacitor circuits, the voltage which is applied is two or three times that which is normally employed, so there is severe damage to the contact surf aces by the arc on current interruption or current switching. This tends to cause surface roughness of the contacts and exfoliative erosion. Since contact erosion is believed to be a cause of restriking, it is also necessary to reduce erosion. However, even though the restriking phenomenon is important from the standpoint of improving product reliability, so far from techniques for preventing it, even its direct causes are still unknown. Ag--WC alloy is employed as a low current chopping characteristic contact material in preference to Cu--Bi alloy, Cu--Te alloy, or Cu--Cr alloy as mentioned above, but the present situation is that it does not provide a fully satisfactory contact material in respect of increasing demands for low current chopping characteristic and low restriking characteristic, and furthermore it is desired to have both these characteristics together to a higher degree. Specifically, even in the case of Ag--WC alloy, which is up to the present preferentially employed as a low current chopping contact material, occurrence of restriking is still observed in circuits having more severe high voltage regions and large rush current. Accordingly, development of a contact material is desired wherein, while maintaining the above-mentioned three basic requirements at a fixed level, in particular both a low chopping characteristics and a good restriking characteristic can be obtained.